Wawa's Boot Camp is an improvement-geared online boot camp, helping to connect coaches with players. They have also created a number of excellent guides.
A well-established tournament for the prize of pride. The Scrub Cup consists of teams of players ranked below gold. Playing on a team is a potentially good way to improve quickly -- you have to learn to work well with others to be successful in such environments.
The Overwatch Community Guide is a wiki-style system geared toward creating a comprehensive guide to many aspects of Overwatch. You can read one of their in-depth guides -- or help our and contribute with your own stuff.
A list of Overwatch-relevant subreddit communities on reddit.com.
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Want to hear about new resources? Suggest changes or updates? Berate me for writing or adding garbage? Come hang out in my Discord. It consists of a channel where I only post updates for this resource thing. It may eventually contain further channels. I don't really know how to work this thing oh god help I need an adult. "This thing" being social interaction, not Discord.